GHCi

GHCi, short for the Glasgow Haskell Compiler interactive environment, is the interactive Read-Eval-Print-Loop (REPL) provided with the Glasgow Haskell Compiler (GHC). It offers a fast, incremental way to develop, test, and explore Haskell code by evaluating expressions, loading modules, and inspecting types without an explicit compilation step.

As a Haskell interpreter built on the GHC backend, GHCi compiles and runs code on demand. It maintains a session that keeps definitions loaded in memory, so later expressions can reference earlier ones. Users can query types with commands such as :type or :t, inspect available names with :info or :browse, and inspect typeclasses. The :load or :reload commands load or reload modules from source files.

Common commands include :set to adjust options, :module to change the active module, and :! to run external shell commands. Expressions entered at the prompt are evaluated lazily, consistent with Haskell semantics, and results are printed to the console. GHCi can also run simple IO actions and test functions interactively.

Starting and using GHCi is straightforward: run ghci at the command line, optionally with a file to preload (for example, ghci MyModule.hs). Once loaded, users interact with the prompt and progressively build and test code. GHCi is widely used for experimentation, debugging, learning, and rapid prototyping, and remains a core tool for Haskell development alongside compiled builds.